Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K08: Non-equilibrium Superconductivity |
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Sponsoring Units: DCMP DMP Chair: Dirk Van Der Marel, University of Geneva Room: BCEC 150 |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K08.00001: Manipulation of Superconductivity through Parametric Driven Phonons Yao Wang, Tao Shi, Valentin Kasper, Juan Ignacio Cirac, Eugene Demler Photoinduced superconductivity has attracted much attention in the past decade, due to the possibility of dramatically enhancing the transition temperature and finally creating a room-temperature superconductor. One promising realization of nonequilibrium superconducting states is achieved via pumping mid-infrared phonons. In this work, we perform a numerical study of an electron-phonon system after dynamically squeezing the phonons, which mimics the leading impact of the infrared pump. Using a variational non-Gaussian wavefunction, we are able to evaluate not only the superconducting order parameter, but also the time-resolved optical and ARPES spectroscopies. By comparing with optical probes, we identify the physical order parameter as a pair of dressed quasiparticles. Driving the system at different frequencies we observe that superconductivity can be either persistently enhanced or suppressed, depending on the resonant characteristic excitations. Revealing the dynamics of the parametric phonon pump, this work enables the design of conventional non-equilibrium superconductors. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K08.00002: Allowing for real coherent phonon bursts to defend and actuate the superconducting state in YBCO John James Assuming that BCS superconductivity is unstable for high transition temperatures, what would be required if real coherent phonon bursts formed to prevent (defend against) the complete collapse of this state when the superconducting gap fell to resonance with the low energy optical soft phonon mode? Could the resonant phonon bursts also establish (actuate) the high energy of the gap by requiring the gap formation energy at very low superconducting state density to be the soft optical phonon energy? For this treatment of the BCS gap as a population inversion to produce coherent phonons, the formation times would usually be enormous compared to the electron-defect scattering times. All defect interactions would need to be eliminated because they would be resonant with the phonon energy but differing in momentum. A model is presented in which chain site, out of CuO2 plane, oxygen atoms tend to mutually repel for nearest neighbor sites while attracting at second nearest neighbor, causes the depletion of chain oxygen in the vicinity of all defects. This forces the fermi level into the band gap near all defects and surfaces and allows the slow coherent phonon emission and superconducting ground state formation to occur. |
Wednesday, March 6, 2019 8:24AM - 8:36AM |
K08.00003: Light-induced d-wave superconductivity through Floquet-engineered Fermi surfaces in cuprates Dante Kennes, Martin Claassen, Michael Sentef, Christoph Karrasch We introduce a mechanism for light-induced Floquet engineering of the Fermi surface to dynamically tip the balance between competing instabilities in correlated condensed matter systems in the vicinity of a van-Hove singularity. We first calculate how the Fermi surface is deformed by an off-resonant, high-frequency light field and then determine the impact of this deformation on the ordering tendencies using an unbiased functional renormalization group approach. As a testbed, we investigate Floquet engineering in cuprates driven by light. We find that the $d$-wave superconducting ordering tendency in this system can be strongly enhanced over the Mott insulating one. This gives rise to extended regions of induced $d$-wave superconductivity in the effective phase diagram in the presence of a light field. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K08.00004: Wavelength Dependence of Pumping a Superconductor Ta Tang, Thomas Devereaux, Brian Moritz Experiment [E. Casandruc et al] has shown transient enhanced superconductivity in LBCO when pumped by ultrafast pulses.This provides a new way to control the propertities of high Tc supercondoctors and explore non-equilibrium phenomenon. Here we explore the effects of pumping the cuprates at different wavelengths. The model is a multi-band Hubbard model (including apical oxygen sites), which we pump with an ultrafast gaussian-pulse to study its time evolution using exact diagonalization (ED). The results show that holes are pumped from the CuO2 plane into the apical oxygens at characteristic optical wavelengths. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K08.00005: Transient superconductivity without superconductivity Giuliano Chiriaco, Andrew Millis, Igor L Aleiner Recent experiments on K3C60 and layered copper-oxide materials have reported substantial changes in the optical response following application of an intense THz pulse. These data have been interpreted as the stimulation of a transient superconducting state even at temperatures well above the equilibrium transition temperature. We propose an alternative phenomenology based on the assumption that the pulse creates a non-superconducting, though non-equilibrium situation in which the linear response conductivity is negative. The negative conductivity implies that the spatially uniform pre-pulse state is unstable and evolves to a new state with a spontaneous electric polarization. This state exhibits coupled oscillations of entropy and electric charge whose coupling to incident probe radiation modifes the reflectivity, leading to an apparently superconducting-like response that resembles the data. Dependencies of the reflectivity on polarization and angle of incidence of the probe are predicted and other experimental consequences are discussed. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K08.00006: Time-dependent McMillan-Ginzburg-Landau models for light-induced ultrafast phase transitions in cuprates and other complex materials Roland Allen, Jian Weng, Ross Tagaras Earlier we found that a very simple time-dependent Ginzburg-Landau model [1] described several features of the light-induced superconductivity in La1.675Eu0.2Sr0.125CuO4 (LESCO1/8) discovered by Cavalleri and coworkers [2]. But for a more detailed description of complex materials -- cuprate and other high-temperature superconductors, layered transition-metal dichalcogenides, colossal magnetoresistive manganites, ... -- more detailed models are needed, which should properly be called McMillan-Ginzburg-Landau models [3]. The description of both static phase diagrams and ultrafast dynamics with such models will be discussed, with a few examples. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K08.00007: Impact of damping on superconducting gap dynamics induced by intense terahertz pulses Tianbai Cui, Xu Yang, Chirag Vaswani, Jigang Wang, Rafael M Fernandes, Peter P. Orth Recent advances in terahertz pump-probe spectroscopy opened a new route to investigate non-equilibrium superconductivity and gap dynamics. The time-dependent BCS theory predicts 2Δ gap oscillations after an ultrafast non-adiabatic perturbation is turned off, a consequence of the excitation of the Anderson-Higgs mode. Because gap relaxation due to the coupling to the phonons happens usually at longer time scales, the average gap is expected to be constant after the pump is off. This is at odds with recent experiments using intense THz pulses with subgap frequencies in NbN, where the gap is seen to be suppressed already at the tens of ps time scale, after the pump is turned off. We show that this behavior arises from damping within the electronic subsystem due to effects beyond BCS theory, such as interactions between Bogoliubov quasiparticles and decay of the Higgs mode. We develop a semi-phenomenological model where these relaxation processes are conveniently expressed as T1 and T2 relaxation times in the pseudospin formalism. We discuss the impact of each relaxation process to the gap dynamics and show the quantitative agreement with the experiments. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K08.00008: Control of Non-equilibrium Quantum Phases and Collective Modes in Superconductors by Terahertz Light-Driven Supercurrents Martin Mootz, Ilias Perakis, Jigang Wang In superconductors several non-equilibrium phases exist ranging from quenched states to gapless superconducting phases with gapless excitation spectrum but unchanged macroscopic coherence and infinite conductivity. The dynamics of such non-equilibrium phases is associated with observation of collective modes which provide details about the underlying nature of superconductivity. Although recent advances in THz laser spectroscopy make the excitation of collective modes and gapless superconducting phases possible, their observations in ultrafast spectroscopy remain challenging. Here we demonstrate control of the non-equilibrium dynamics in superconductors by THz light-driven supercurrents. By applying a microscopic gauge-invariant theory we show that selective non-equilibrium phases can be excited and amplitude Higgs mode can be detected in the nonlinear response by inducing supercurrents via THz pulse shaping. THz light-control of supercurrents also allows for generation of a comb of odd or odd and even harmonics in emission. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K08.00009: Collective modes in superconductors and their coupling to near field THz probes Zhiyuan Sun, Michael Fogler, Andrew Millis We theoretically investigate methods to observe the phase (Anderson-Bogoliubov-Goldstone), amplitude (Higgs) and Carlson-Goldman modes using THz near field techniques. In a quasi-two-dimensional superconductor, we show that the Carlson-Goldman mode appears in the near field reflection coefficient as a weak feature in the sub-Terahertz frequency range. In a system of two superconductor layers separated by nanometer scale, the gapless phase mode reappears due to the mutual screening between the layers, as the antisymmetric plasmon mode of the system. This mode leads to a well defined resonance peak in the near-field THz response. Close to zero temperature, its speed contains the information of the original Goldstone mode. The amplitude mode could appear in the near field third harmonic generation for particle-hole asymmetric superconductors and we also discuss its observability in the linear response. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K08.00010: The Higgs Amplifier Daniel Podolsky, Yao Wang, Eugene Demler A sudden quench into a superconducting state results in strong Higgs oscillations in the amplitude of the superconducting order parameter. We study theoretically the optical response of a quenched superconductor and find that Higgs modulations lead to a marked enhancement of the reflectance. For strong enough modulations, the reflectance can even exceed unity, corresponding to signal amplification. We discuss possible implications to experiments on light-induced superconductivity in K3C60. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K08.00011: Collective modes in non-equilibrium in unconventional superconductors with competing ground states Marvin Alexander Müller, Pavel Volkov, Ilya Eremin
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Wednesday, March 6, 2019 10:12AM - 10:24AM |
K08.00012: Control of Competing Superconductivity and Charge Order By Non-equilibrium Currents Anne Matthies, Jiajun Li, Martin Eckstein Strongly correlated materials often have rich phase diagrams. However, in many cases potentially interesting states are suppressed by competing phases which are thermodynamically in close proximity, e.g. superconductivity (SC) and charge-density-wave (CDW) order. How to control and distinguish competing phases arises as a very interesting and relevant question which is currently under intense research. In this work we study the competing CDW and SC order in the attractive Hubbard model under a voltage bias, using non–equilibrium steady-state (NESS) dynamical mean-field theory. We show that the CDW is suppressed in a current-carrying NESS by an effect beyond a simple Joule-heating mechanism. A “supercooled” metallic state is stabilized at a NE temperature lower than the equilibrium SC Tc. Since a current-carrying SC state is dissipation-less and thus not subject to the same non-thermal suppression, it can nucleate out of the supercooled metal. Hence, an electric current can change the relative stability of different phases compared to thermal equilibrium, even when a system appears locally thermal due to electron-electron scattering. This provides a general perspective to control intertwined orders out of equilibrium. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K08.00013: Efficient Prediction of Time-and Angle-Resolved Photoemission Spectroscopy Measurements on a Non-Equilibrium BCS Superconductor Tianrui Xu, Takahiro Morimoto, Alessandra Lanzara, Joel Moore We study how time-and angle-resolved photoemission (tr-ARPES) reveals the dynamics of BCS-type, s-wave superconducting systems with time-varying order parameters. Approximate methods are discussed, based on previous approaches to either optical conductivity or quantum dot transport, in order to enable computationally efficient prediction of photoemission spectra. One use of such predictions is to enable extraction of the underlying order parameter dynamics from experimental data, which is topical given the rapidly growing use of tr-ARPES in studying unconventional superconductivity. The methods considered model the two-time lesser Green's functions with an approximated lesser self-energy that describes relaxation by coupling of the system to two types of baths. The approach primarily used here also takes into consideration the relaxation of the excited states into equilibrium by explicitly including the level-broadening of the retarded and advanced Green's functions. We present equilibrium and non-equilibrium calculations of tr-ARPES spectrum from our model and discuss the signatures of different types of superconducting dynamics. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K08.00014: Probing Out-of-Equilibrium Spin Modes in a Mesoscopic Superconductor Marko Kuzmanović, BIYI WU, Maximilian Weideneder, Charis H.L. Quay, Marco Aprili While energy and charge-mode excitations in a superconductor (associated to out-of-equilibrium Bogoliubov quasiparticles) are well known, spin modes have been addressed only recently. If spins are injected into a mesoscopic superconductor, a long-range out-of-equilibrium magnetization can be observed [1], and both the spin accumulation [2] and spin relaxation [3] times have been measured, being ~10-20 ns and ~50 ps respectively. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K08.00015: Optimal probabilistic work extraction beyond the free energy difference with a single-electron device Olivier Maillet, Paolo Erdman, Vasco Cavina, Bibek Bhandari, Elsa Mannila, Joonas T Peltonen, Andrea Mari, Fabio Taddei, Christopher Jarzynski, Vittorio Giovannetti, Jukka P Pekola We experimentally realize protocols that allow to extract work beyond the free energy difference from a single electron transistor at the single thermodynamic trajectory level. With two carefully designed out-of-equilibrium driving cycles featuring kicks of the control parameter, we demonstrate work extraction up to large fractions of the thermal/charging energy or with probabilities substantially greater than 1/2, despite zero free energy difference over the cycle. Our results are explained in the framework of nonequilibrium fluctuation relations. We thus show that irreversibility can be used as a resource for optimal work extraction even in the absence of feedback, as opposed to a traditional Maxwell's Demon. |
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